Field of the Invention
The invention relates to a cam-controlled power differential gear for a sheet acceleration system, which is formed of a pregripper having an acceleration course determinable by a control cam driven at a single speed.
In sheet-fed rotary printing presses, it is customary for a sheet oriented on a feeding table to be gripped by a sheet acceleration system formed as a pregripper, and accelerated up to processing speed. After transferring the sheet to a sheet transport drum, the pregripper is decelerated down to a standstill and then is reaccelerated in the direction of the feeding table, with a subsequent deceleration down to a standstill at the feeding table. Because of the cyclical motion of the sheet acceleration system over a machine cycle, forces of inertia are generated which create torques acting upon the control cam; these torques are superimposed upon the torques operative in the drive system and thus cause torque fluctuations which, in the final analysis, cause registration errors and doubling or double impressions, respectively, which consequently lead to defective quality or a rejection of the printed products. The torque fluctuations also cause increased wear of the printing press.
From the published German Patent Document DE 41 09 824 A1, a cam-controlled power differential gear has become known heretofore which minimizes the aforedescribed problems. To that end, four compensating masses are provided, offset from one another by 90.degree., which engage with a common compensating cam. Rollers respectively assigned to one compensating mass pass per period, i.e., one reciprocating pivoting motion of the pregripper, through two identical motion segments without a resting phase at a sheet transfer location on the feeding table. The length of the resting phase is in fact determined solely by the reversal point of the pregripper on the feeding table. This very severely restricts the free choice of a pregripper motion principle.